Cryptococcus neoformans is a common cause of fatal infection in patients with AIDS. The respiratory tract is the portal of entry for C. neoformans. The development of protective Th1 cell-mediated immunity (CMI) is crucial to the eradication of the organism and control of cryptococcal dissemination from the lungs to the brain. However, C. neoformans can also stimulate a Th2 type response, resulting in a non-resolving, chronic pulmonary infection. The production of Th2 type cytokines during C. neoformans infection may allow the organism to establish a chronic pulmonary infection from which dissemination could occur during advance stages of T cell deficiency as found in AIDS. Our preliminary studies indicate that TNF alpha production is critical or the development of Th1 immunity and the interplay between hosts TNF alpha and microbial (melanin) factors is a major determinant for Th1 vs. Th2 immunity to C. neoformans. The hypothesis of this proposal is that alveolar macrophage production of TNF alpha and the onset of infection promotes the development of Th1 CMI, while the cryptococcal virulence factor promotes the switch to Th2 CMI. The specific aims are to determine: (1) the relationship between the timing of TNF alpha production in the lungs and the induction of Il-12, interferon gamma and Th1 type immunity; (2) whether the protective effect of intratracheally administered TNF alpha peptide during pulmonary C. neoformans infection is due to induction of Th1 type CMI in the lungs; (3) whether depletion of alveolar macrophages prior to C. neoformans infection ablates TNF alpha induction and prevents the development of Th1 type CMI; (4) whether the development of C. neoformans-specific Th1 vs. Th2 cells is dependent on the melanin content of C. neoformans during immunization; and (5) whether the synthesis of melanin in vivo by C. neoformans promotes the development of Th2 type CMI. This proposal will investigate whether TNF alpha production by alveolar macrophages is a critical factor for Th1/Th2 switching in the lungs and functions by promoting a Th1 response. This proposal will also investigate the concept that live microbes are dynamic antigens, such that melanin production modulates the development of Th1 vs. Th2 immunity by altering the microenvironment at the time of immune recognition. One of the potential implications of these studies is that therapies inhibiting melanin biosynthesis or activity may also have an adjuvant effect and promote the development of protective Th1 immunity to C. neoformans.